Patent GB-1288679-A (GB)
IPC: G03B7/08
Filed: 1970-06-15
Abstract: 1288679 Automatic exposure time control CANON KK 15 June 1970 [13 June 1969 16 July 1969 21 July 1969 18 Nov 1969 (4) 1 Dec 1969 19 Dec 1969 (2) 12 Jan 1970 13 Feb 1970] 28821/70 Heading G2A [Also in Divisions G1 and H3] An exposure time control circuit has a photoresistor 1, an amplifier 4, 5, and a regenerator circuit 6, 7, 8 which receives an input from the amplifier output and provides an input both to the amplifier and to an exposure time determining capacitor 10. As shown, Fig. 2, the photoresistor is in series with a log-diode 2 to provide a voltage E 1  representative of scene illumination which is connected to one input 4 of a differential amplifier via a signal storage capacitor 3. The other input 5 of the differential amplifier receives an input E 2  across an identical log-diode 6 via transistors 7, 8. Under steady conditions E 1  = E 2  and the emitter-collector resistance of transistor 8 is equal to the resistance of the photo-resistor. The output is coupled to a further transistor 9 which operates as a resistor element in the shutter speed timing circuit 10, 14, 15. At shutter operation, switch 18 is open and switches 17, 20 are closed. Further movement of the shutter release opens switches 17 and 19, and the exposure time is determined by the voltage stored on capacitor 3. In modifications: Fig.3 (not shown) log-diodes 2, 6 are replaced by one logdiode (22) which is switched either into the photometry circuit or the regenerator circuit, capacitor 10 is adjustable according to film speed or diaphragm aperture, and a photo-conductor (23) may be placed in series with capacitor 10 to operate the shutter control independently of the regenerator circuit; Fig. 4 (not shown) the photosensitive element is a photo-diode and is connected to the amplifier input via a transistor (28) having a variable base potential; Fig. 6 (not shown) greater temperature and supply voltage stabilization is achieved by providing a thermistor (32) at the drain side of field effect transistor 5 and a resistor (33) at the emitter side of transistor 7, and a resistor (30) is varied according to the set diaphragm value and is connected at the emitter side of transistor 11; Figs. 10, 11 and 12 (not shown) include an ammeter (34) or a voltmeter (35) to give a reading in a viewfinder and a warning of possible over- or under-exposure; Fig. 14 (not shown) a variable resistor (37) may be connected in place of photo-resistor, or a variable resistor (38) may be connected to capacitor 10 in place of the regenerator circuit, to provide manual control of exposure time; and Figs. 15 and 16 (not shown) provide self-holding of the power supply switch. As shown, Fig. 17, variable transmittance elements, e. g. liquid crystals or an electrolytic cell, are used. Photo-resistor 53 which is connected to the exposure timing capacitor 54 is illuminated through element 51 whose transmittance is adjusted according to set diaphragm value, and through element 48 whose transmittance is varied according to the voltage stored on capacitor 3. In still further embodiments; Fig.19 (not shown) transistor 9 is omitted and a retainer capacitor (99) is provided at the input to amplifier input 5, transistor 8 being connected either to the amplifier input 5 or to controlling capacitor 10; Fig. 20 (not shown) four variable resistances form a bridge circuit with the photo-resistor bridging one arm, a differential amplifier being connected to the output points, and an ammeter indicates the shutter speed and also is used as a battery tester. In other embodiments, photographic information is set using circuitry having log-diodes, and a filter may be positioned in front of the photo-resistor to maintain linearity at high intensity levels, the filter being inserted under the control of a further photo-resistor.